In recent years, there has been rapid progress in portable and/or cordless designs of audiovisual equipment, personal computers and other electronic equipment. As the power supply for these electronic equipment, high capacity type of non-aqueous electrolyte secondary batteries such as represented by various alkaline storage batteries and lithium secondary batteries are suitable. These non-aqueous electrolyte secondary batteries are desired to be implemented as sealed batteries with a high energy density and with superior load characteristics.
On the other hand, sealed batteries with a high energy density tend to generate abnormal gas inside the cells due to chemical reaction resulting from troubles in associated equipment including the charger or from over charge or erroneous use, thus causing an excessive pressure inside cells, explosion of cells, or damage to the electronic equipment using the cells as the power supply.
In order to prevent these accidents, these types of cells have hitherto been equipped with a safety device against explosion to allow gas to exhaust by opening a vent when the cell internal pressure exceeds a preset value. Furthermore, as non-aqueous electrolyte secondary batteries have a danger of ignition due to a rapid temperature rise, they are equipped with a reliable safety mechanism against explosion which will completely cut off electric current prior to exhausting gas by detecting the internal pressure.
As an example, in Japanese Laid-Open Patent No. Hei 6-196150, a mechanism is disclosed wherein a vent on top of a cell and a terminal plate having a vent hole are made electrically conductive through their central welded section, and when the internal pressure rises to a predetermined value, the vent which is exerted with the pressure through the vent hole of the terminal plate will be detached from the section welded with the terminal plate by an outwardly deforming stress due to the pressure through the vent hole of the terminal plate, thereby cutting off an electric current.
In the above-described explosion-proof safety mechanism, ultrasonic welding which is capable of welding to a low weld strength is employed in welding the vent and the terminal plate because of the necessity of welding necessary portions of the vent and the terminal plate to a weld strength low enough to allow detachment at a certain internal pressure. However, since ultrasonic welding causes fusion by vibration heating only on the surface of the work piece, there is a possibility of causing a large dispersion in the weld strength.
Consequently, in the above-described explosion-proof safety mechanism, as the pressure of cutting off an electric current is determined dependent on the weld strength of the welded portion, the pressure of cutting off an electric current varies with the variability of the weld strength, exhibiting a drawback of not being able of setting to a fixed value. As a result, it suffers problems of cutting off an electric current before the cell internal pressure rises to a predetermined value, or not cutting off an electric current even when the cell internal pressure has risen to a predetermined value.
Therefore, a higher-accuracy method of cutting off an electric current has become necessary which is not affected by the weld strength in cutting off an electric current.